feat(go): expose fast writer batch pending metrics

docs/ops/100k-capacity-baseline.md

@@ -76,6 +76,8 @@ systemd gateway 已配置 `LimitNOFILE=1048576`，需要持续保持。
 | NATS bridge | `vehicle_bridge_nats_consumer_pending` | burst 后下降 |
 | NATS bridge | `vehicle_bridge_batch_pending_messages` | 稳态接近 0，burst 后下降 |
 | NATS bridge | `vehicle_bridge_batch_duration_ms_histogram_bucket` | p99 不持续上升 |
+| NATS fast writer | `vehicle_fast_writer_batch_pending_messages` | 稳态接近 0，burst 后下降 |
+| NATS fast writer | `vehicle_fast_writer_batch_pending_envelopes` | 稳态接近 0，burst 后下降 |
 | NATS fast writer | `vehicle_fast_writer_stage_duration_ms_histogram_bucket` | TDengine/Redis/ack 各阶段 p99 不持续上升 |
 | Kafka consumers | `vehicle_*_kafka_lag` | 稳态为 0，burst 后下降 |
 | Realtime Redis | `vehicle_realtime_store_update_duration_ms_histogram_bucket{store="redis"}` | p99 毫秒级且不持续上升 |

docs/ops/go-service-observability.md

@@ -47,6 +47,8 @@ curl -fsS http://127.0.0.1:20200/readyz
 | `vehicle_bridge_batch_pending_messages` | NATS messages fetched by the bridge but not yet written to Kafka and acked. |
 | `vehicle_bridge_batch_pending_kafka_messages` | Kafka messages prepared for the current bridge batch. |
 | `vehicle_bridge_batch_duration_ms_histogram` | Bridge batch duration histogram covering Kafka write and NATS ack. Labels: `status`. |
+| `vehicle_fast_writer_batch_pending_messages` | NATS fast writer messages fetched but not yet written to TDengine/Redis and acked. |
+| `vehicle_fast_writer_batch_pending_envelopes` | Valid parsed envelopes in the current NATS fast writer batch. |
 | `vehicle_fast_writer_stage_duration_ms_histogram` | Fast writer stage duration histogram for TDengine, Redis, and NATS ack. Labels: `subject`, `stage`, `status`. |
 | `vehicle_history_writes_total` | TDengine history writes. Labels: `topic`, `status`. |
 | `vehicle_history_batch_pending_messages` | Messages fetched by history writer but not yet flushed and committed. |

docs/ops/go-vehicle-ingest-memory.md

@@ -423,6 +423,15 @@ vehicle_fast_writer_stage_duration_ms_histogram_sum{subject,stage,status}
 
 2026-07-03 后续优化：`nats-fast-writer` 已从“Fetch batch 后逐条写 TDengine”改为“Fetch batch 后先调用 `history.Writer.AppendAllBatch` 批量写 TDengine，再逐条更新 Redis 和 ack”。这样可以减少 TDengine round trip；Redis/ack 仍按单条执行，避免某条实时投影失败时误 ack。
 
+同时新增 batch pending gauge：
+
+```text
+vehicle_fast_writer_batch_pending_messages
+vehicle_fast_writer_batch_pending_envelopes
+```
+
+这两个指标表示 fast-writer 已拉取但尚未完成 TDengine/Redis/ack 的批内压力。带帧率压测时它们应在稳态接近 0，burst 后能够回落；如果持续非 0，需要结合 `vehicle_fast_writer_stage_duration_ms_histogram` 判断卡在 TDengine、Redis 还是 ack。
+
 ### NATS Kafka bridge batch 指标
 
 2026-07-03 已为 NATS -> Kafka bridge 增加 batch pending 和 duration histogram：

docs/ops/vehicle-ingest-runbook.md

@@ -129,6 +129,8 @@ go run ./cmd/load-sim \
 | `vehicle_bridge_nats_consumer_pending` | 持续增长且 `> 10000` | bridge 消费 NATS 的速度跟不上生产速度。 |
 | `vehicle_bridge_batch_pending_messages` | 持续非 0 或 burst 后不回落 | bridge 已拉取 NATS 消息但尚未完成 Kafka 写入和 NATS ack。 |
 | `vehicle_bridge_batch_duration_ms_histogram_bucket` | p99 连续 5 分钟上升 | Kafka 写入或 NATS ack 开始变慢，通常会先于 ack-pending 扩大。 |
+| `vehicle_fast_writer_batch_pending_messages` | 持续非 0 或 burst 后不回落 | fast-writer 已拉取 NATS 消息但尚未完成 TDengine/Redis 写入和 ack。 |
+| `vehicle_fast_writer_batch_pending_envelopes` | 持续非 0 或 burst 后不回落 | fast-writer 当前批次已有有效 envelope 在等待落库或 ack。 |
 | `vehicle_fast_writer_stage_duration_ms_histogram_bucket` | 某个 stage 的 p99 连续 5 分钟上升 | NATS 快速写链路在 TDengine、Redis 或 NATS ack 某一阶段变慢。 |
 | `vehicle_realtime_store_update_duration_ms_histogram_bucket{store="redis"}` | p99 连续 5 分钟上升 | Redis 实时投影变慢，会直接影响 realtime consumer 追平能力。 |
 | `vehicle_realtime_store_update_duration_ms_histogram_bucket{store="mysql"}` | p99 连续 5 分钟上升 | MySQL 当前态/位置投影变慢，需结合 async queue depth 和 dropped 计数判断。 |